1. Field of the Invention
The present invention relates to a dispersion management optical transmission system and optical transmission line, more particularly relates to a dispersion management optical transmission system for high speed wavelength division multiplexing (WDM) transmission and a WDM optical transmission line comprising that dispersion management optical transmission system (optical fiber).
2. Description of the Related Art
With optical transmission becoming faster in speed and greater in capacity, WDM transmission is being looked at closely as a mainstream technology.
The new problem of non-linear phenomena due to the higher power of optical signals and the interaction between signal wavelengths has arisen, however.
Among the non-linear phenomena, four wave mixing (FWM) has a serious effect at the time of WDM transmission. Methods to suppress it are being actively studied.
FWM easily occurs in region of small dispersion, so a small dispersion fiber is known to be disadvantageous in that point. Even the conventionally used non-zero dispersion shift fiber (NZxe2x88x92DSF) has been insufficient in this regard.
Further, the waveform distortion due to self-phase modulation (SPM) or cross-phase modulation (XPM) etc. is also a serious problem. In the research to solve this, along with studies to keep down the non-linear refractive index (n2) reported in OFC"" 97 TuN1b etc., the enlargement of the effective core area (Aeff) is extremely interesting.
The distortion xcfx86NL of a signal due to non-linear phenomena is generally expressed by the following formula (1):
xcfx86NL=(2xcfx80xc2x7n2xc2x7Leffxc2x7P)/(xcexxc2x7Aeff)xe2x80x83xe2x80x83(1)
where, n2 (m2/W): non-linear refractive index,
Leff (m): effective length,
P (W): power,
xcex (nm): wavelength, and
Aeff (xcexcm2): effective core area.
From formula (1), it is learned that it is advantageous that the effective core area (Aeff) be large.
As reported in OFC""96 WK15 or OFC"" 97 TuN2, the enlargement of the effective core area (Aeff) is one of the most sought after features.
Enlargement of the effective core area (Aeff), however, is known to easily cause problems such as an increase in the bending loss or an increase in the dispersion slope in a fiber of a type such as the conventional NZxe2x88x92DSF used for forming a transmission line by itself. This is also a problem in NZxe2x88x92DSF types.
To solve the above problems, it is reported that the method of managing the dispersion for the line as a whole is effective.
For example, Japanese Unexamined Patent Publication (Kokai) No. 9-211511 discloses to obtain an optimal line by connecting two fibers of opposite positive and negative dispersions in the order of the less non-linear (smaller n2/Aeff) fiber and the more non-linear (larger n2/Aeff) fiber.
As a specific example, as shown in ECOC"" 97 vol. 1, p. 127, it is proposed to use a combination of a single mode optical fiber (SMF) having positive dispersion characteristics and a dispersion compensation optical fiber (DCF) having negative dispersion characteristics.
A 1.31 zero-dispersion single mode optical fiber, that is, a fiber having zero dispersion at a wavelength of 1.31 xcexcm, has very superior properties in the points of the non-linearity and transmission loss, but has a large positive dispersion and dispersion slope at a wavelength of the 1.55 xcexcm band. With this SMF alone, it is not possible to transmit light of the 1.55 xcexcm band without dispersion, so compensation of the dispersion becomes necessary and therefore a DCF having the above negative dispersion characteristic is combined.
Such a DCF is connected with the SMF and used as a line rather than the conventional modular type dispersion compensation fiber, so is called a xe2x80x9cline use dispersion compensation fiberxe2x80x9d. Further, it has a negative dispersion (reverse dispersion), so is called a xe2x80x9creverse dispersion fiber (RDF)xe2x80x9d.
The above SMF and RDF are managed to obtain a zero dispersion in total in the 1.5 xcexcm band, but each fiber has a large dispersion of about 16 to 22 ps/nm/km in absolute value in the 1.5 xcexcm band. This is advantageous in terms of suppressing FWM.
Further, an RDF is a fiber designed to cancel out the dispersion and dispersion slope of an SMF and can therefore achieve a dispersion flatness suitable for WDM transmission in the line as a whole.
The dispersion compensation performance when connecting an SMF and RDF can be easily understood by expression by the compensation rate CR shown by the following formula (2) for example:
CR(%)=[(SlopeRDF/SlopeSMF)/(DispersionRDF/DispersionSMF)]xc3x97100xe2x80x83xe2x80x83(2)
In formula (2), xe2x80x9cSlopeRDFxe2x80x9d indicates the dispersion slope (ps/nm2/km) of the RDF, xe2x80x9cSlopeSMFxe2x80x9d indicates the dispersion slope (ps/nm2/km) of the SMF, xe2x80x9cDispersionRDFxe2x80x9d indicates the dispersion (ps/nm/km) of the RDF, and xe2x80x9cDispersionSMFxe2x80x9d indicates the dispersion of the SMF.
From formula (2), the nearer the compensation rate CR to 100%, in the wider wavelength range zero dispersion can be realized. That is, the closer the dispersion per slope (DPS) values of the SMF and RDF, the lower the dispersion slope obtained. The RDF has a high compensation characteristic of a DPS of about 300 (nm).
However, an RDF or other dispersion compensation type fiber has a larger non-linearity than an SMF etc., so when connecting an SMF and RDF by a ratio (length ratio) of 1:1, relatively large power is incident into the RDF and XPM and other non-linear phenomena can no longer be avoided.
Therefore, recently, as shown in OECC"" 9815C1-3, attempts have been made to shorten the ratio of connection of the RDF to SMF and suppress the non-linear phenomena by connecting an SMF having a dispersion value of 16 to 22 ps/nm/km and a high dispersion RDF having a larger absolute value of dispersion than the SMF. In such a combination, however, the total characteristics end up deteriorating compared with a conventional NZxe2x88x92DSF
In this way, NZxe2x88x92DSF and SMF+RDF system suffer from above problems.
An object of the present invention is to provide a dispersion management optical transmission system suppressing the occurrence of non-linear phenomena and reducing the transmission loss and an optical transmission line using the same.
According to a first aspect of the present invention, there is provided a dispersion management optical transmission system comprising a first optical fiber having a positive dispersion and a positive dispersion slope in the 1.5 xcexcm band and a second optical fiber or a second optical fiber module connected to a rear side of the first optical fiber and having a negative dispersion and a negative dispersion slope in the 1.5 xcexcm band, the dispersion of the first optical fiber in the 1.55 xcexcm band being 8 to 15 ps/nm/km and the dispersion slope being at least 0.04 ps/nm2/km, the dispersion of the second optical fiber or the second optical fiber module in the 1.55 xcexcm band being not more than xe2x88x9240 ps/nm/km and the dispersion slope being not more than xe2x88x920.08 ps/nm2/km, the cumulative dispersion of the first optical fiber being at least 200 ps/nm, and the cumulative dispersion when combining the first optical fiber and the second optical fiber or second optical fiber module being suppressed in any wavelength region of the 1.5 xcexcm band.
Here, a xe2x80x9c1.5 xcexcm bandxe2x80x9d is for example a range of wavelength of 1.45 to 1.65 xcexcm.
Preferably, the average dispersion when combining the first optical fiber and the second optical fiber or second optical fiber module is within xc2x13 ps/nm/km at any wavelength region of the 1.5 xcexcm band.
More preferably, the length of the first optical fiber is at least four times the length of the second optical fiber or the second optical fiber module.
Still more preferably, a transmission loss of the first optical fiber in the 1.55 xcexcm band is not more than 0.21 dB/km, a transmission loss of the second optical fiber or the second optical fiber module in the 1.55 xcexcm band is not more than 0.50 dB/km, and a total transmission loss in the 1.55 xcexcm band when combining the first optical fiber and the second optical fiber or second optical fiber module, excluding a connection loss, is not more than 0.300 dB/km.
Alternatively, a transmission loss of the first optical fiber in the 1.55 xcexcm band is not more than 0.20 dB/km, a transmission loss of the second optical fiber or the second optical fiber module in the 1.55 xcexcm band is not more than 0.27 dB/km, and a total transmission loss in the 1.55 xcexcm band when combining the first optical fiber and the second optical fiber or second optical fiber module, excluding a connection loss, is not more than 0.215 dB/km.
Still more preferably, the dispersion slope in the 1.55 xcexcm band when combining the first optical fiber and the second optical fiber or second optical fiber module is xe2x88x920.030 to 0.030 ps/nm/m2/km.
Still more preferably, the effective core area (Aeff) of the first optical fiber is at least 70 xcexcm2.
Alternatively, the effective core area (Aeff) of the first optical fiber is at least 90 xcexcm2.
Still more preferably, the effective core area (Aeff) of the second optical fiber or second optical fiber module is at least 20 xcexcm2.
Still more preferably, the bending loss at a diameter of 20 mm of the second optical fiber or second optical fiber module in the 1.55 xcexcm band is not more than 5 dB/m.
Still more preferably, the first optical fiber is configured having a core and a cladding, the core having a center core which has a refractive index higher than the cladding, and a side core formed around the outer circumference of the center core and having a refractive index between a refractive index of the center core and a refractive index of the cladding.
Still more preferably, the second optical fiber or second optical fiber module is configured having a core and a cladding, the core having a center core which has a refractive index higher than the cladding, a first side core formed around the outer circumference of the center core and having a refractive index lower than the cladding, and a second side core formed around the outer circumference of the first side core and having a refractive index higher than a refractive index of the cladding.
Still more preferably, in the second optical fiber or second optical fiber module, a relative refractive index difference (xcex941) of the center core with respect to the cladding is 1.2 to 1.5%, a parameter a at the time of approximation by the following formula (3) for a refractive index profile (n(r)) of the center core and a distance r from the center of the optical fiber being 2 to 5,                               n          ⁡                      (            r            )                          ∝                  {                      1            -                          2              ⁢                                                Δ                  ⁡                                      (                                          r                      a                                        )                                                  α                                              }                                    xe2x80x83            
where, a is the center core radius and
xcex94 is (n1xe2x88x92n2)/n1 (n1 is the maximum refractive index of the center core, while n2 is the refractive index of the cladding) a relative refractive index difference (xcex942) of the first side core with respect to the cladding is xe2x88x920.50 to xe2x88x920.35%, a relative refractive index difference (xcex943) of the second side core with respect to the cladding is 0.15 to 0.30%, and a ratio of the center core radius (a): first side core radius (b): second side core radius (c) is 1:1.7 to 2.0:2.4 to 3.0.
According to a second aspect of the present invention, there is provided an optical transmission line provided with at least one of any of the above dispersion management optical transmission systems.
According to a third aspect of the present invention, there is provided an optical transmission line provided with a plurality of any of the above dispersion management optical transmission systems.